Add support to dart2js for option --enable-asserts.

pkg/expect/lib/expect.dart

Index: pkg/expect/lib/expect.dart
diff --git a/pkg/expect/lib/expect.dart b/pkg/expect/lib/expect.dart
index 8e312063943e96fc9f47f741e98234673b6e60c4..d25e3e065517a2274164bcc56b27955ce62a5874 100644
--- a/pkg/expect/lib/expect.dart
+++ b/pkg/expect/lib/expect.dart
@@ -503,3 +503,28 @@ class TrustTypeAnnotations {
 class AssumeDynamic {
   const AssumeDynamic();
 }
+
+/// Is true iff type assertions are enabled.
+final bool typeAssertionsEnabled = (() {
+  try {
+    var i = 42;
+    String s = i;
+  } on TypeError catch (e) {
+    return true;
+  }
+  return false;
+})();
+
+/// Is true iff `assert` statements are enabled.
+final bool assertStatementsEnabled = (() {
+  try {
+    assert(false);
+  } on AssertionError catch (e) {
+    return true;
+  }
+  return false;
+})();
+
+/// Is true iff checked mode is enabled.
+final bool checkedModeEnabled =
+    typeAssertionsEnabled && assertStatementsEnabled;